Regional cerebral blood flow (rCBF) will be measured via 133Xenon inhalation in groups of patients with depression and schizophrenia and matched normal controls. The study is aimed at identifying patterns of rCBF associated with these illnesses and their sub types. Ninety patients with a diagnosis of depression and their controls will be studied over a period of two years. rCBF measurement and rating scale quantification of psychopathology will be carried out. A small subgroup of twenty-five patients will be placed on a standardized treatment schedule to gather pilot data on the effect of treatment on rCBF. On hundred and twenty patients with a diagnosis of schizophrenia and their controls will have their cerebral blood flow measurement and rating scale assessment of psychopathology carried out. This study will take place over a period of three years. Like in the case of depressed patients, a subgroup of patients will be placed on a standardized treatment schedule to gather pilot information about the effect of treatment on rCBF.